During neural development, axons extend through an environment containing a multitude of guidance molecules that influence their growth and direction. There is still relatively little known about how axonal growth cones are guided in the complex in vivo environment. We are interested in the particular axon guidance question of how two axon branches from one neuron are guided along separate pathways to distinct targets. Neurons often project axons to more than one target, and virtually nothing is known about how their axon branches are differentially guided. We use as a model zebrafish spinal sensory neurons, which are a simple neuronal population with well-characterized axon trajectories. These neurons extend two axon branches, the central and peripheral axons, that display very different behaviors and trajectories. The goal of this project is to investigate mechanisms of guidance of these axons along their particular pathways and to elucidate the function of the molecular cues that guide them. The zebrafish embryo is an excellent system to investigate axon guidance mechanisms in vivo because we can readily manipulate guidance molecules and image effects on dynamic behavior of axonal growth cones in the intact living embryo. Our specific aims are: 1) to use live imaging in vivo to characterize the behavior of the peripheral axon as it initially extends in an orthogonal direction to the central axon and exits the spinal cord, and investigate the roles of two molecules known to be important for peripheral axon extension; 2) to identify and analyze the function of new molecules involved in sensory axon guidance; and 3) to characterize and clone the mutated gene from a mutant that has defective sensory axon pathways. An understanding of the complex processes involved in promoting and inhibiting axon growth, and the functions of molecules that guide axons to their correct targets will be crucial for understanding diseases of neural development. Moreover, knowledge of these mechanisms will help to understand the conditions under which axon regeneration after injury can occur.